Process for warp-free pigmenting of polyolefins

ABSTRACT

The instant invention relates to a process for the warp-free pigmenting of polyolefins, which process comprises admixing coated pigment particles with a polyolefin, wherein said coated pigment particles are prepared by coating organic pigment particles with one or more films of a preformed polymer by adsorption thereon of from 1 to 10% by weight of the polymer, based on the pigment particles, at room temperature, the polymer being selected from the group consisting of polyvinylpyrrolidone homopolymers and copolymers.

This is a continuation of application Ser. No. 08/121,086, filed Sep.14, 1993, abandoned, which is a continuation of application Ser. No.07/931,751, filed Aug. 18, 1992, abandoned, which is a continuation ofapplication Ser. No. 07/726,055, filed Jul. 5, 1991, now U.S. Pat. No.5,274,010.

The present invention relates to a process for the warp-free pigmentingof polyolefins using organic pigments coated by polar polymers.

In the colouring of polyolefins, in particular polyethylene, usingorganic pigments, it is frequently observed that a number of mechanicalproperties of the coloured material changes with time; defects occurwhich are collectively known as warping phenomena. In large-volumeinjection-moulded parts, such as bottle crates, made from high densitypolyethylene, for example, deformation and shrinkage can be observed,sometimes resulting in cracking and usually rendering the articlesunusable. Frequently, embrittlement of the material also sets in after arelatively short service life. These disadvantages relate to themajority of organic pigments, while inorganic pigments and a minority oforganic pigments have a neutral behaviour. It is assumed that thesedisadvantages arise because the typical crysllisation behaviour of thepolyolefins is disturbed by the presence of the pigments, since thelatter can act as centres of nucleation.

Various methods have already been proposed for overcoming thesedisadvantages. These mostly relate to certain classes of pigments. Forexample, warp-free colouring of polyolefins is achieved, according toU.S. Pat. No. 4,233,206, by using organic pigments containing methylolgroups which have been esterified by carboxylic acids (in particularcopper phthalocyanines); according to JP Kokai 53/124 555, U.S. Pat. No.4,107,133 and JP Kokai 53/143 646 by using an isoindolinone or copperphthalocyanine pigment which has been treated with a silane coupler;according to JP Kokai 53/132 048 by using an isoindolinone pigment withaddition of a salicyloyl hydrazide; according to JP Kokai 57/73 029 byusing a quinacridone, isoindolinone, azo or phthalocyanine pigment whichis adsorbed onto inorganic material (for example silica gel) and thentreated with a silane or titanium coupler, according to JP Kokai 57/159831 by using an isoindolinone pigment which has been treated with aphenylenebistetrachlorobenzamide; and according to JP Kokai 58/23 840 byusing a phthalocyanine pigment with addition of a halomethylcopperphthalocyanine which has been treated with an amine.

According to JP Kokai 57/51 733 and 57/155 242, the warp properties ofisoindolinone pigments can also be improved by recrystallising thepigment. However, as is generally known amongst those skilled in theart, the increase in mean particle size which this produces is at thecost of other important pigment properties, in particular the colourstrength.

U.S. Pat. No. 4,049,615 describes a process for the warp-free colouringof polyolefins using an organic pigment, in particular a perylene,perinone, bisazo or isoindolinone pigment, coated with amelamine-formaldehyde resin.

These methods do not always meet the high demands of modern industryand, depending on the pigment, do not always give the desired result.Thus, for example, no method is known which would allowdiketopyrrolopyrrole pigments to be used for the warp-free pigmenting ofpolyolefins.

The coating of pigments with polar polymers is known per se. Thus, forexample, U.S. Pat. No. 3,806,464 discloses pigments for example copperphthalocyanine, quinacridone, azo, thioindigo and flavanthrone pigments,whose particles are coated with a certain acrylic polymer compositionwhich pigments, for compatibility reasons, are particularly suitable forcolouring the acrylic finishes which are increasingly employed today inthe automotive industry.

DE-A-2 200 322 discloses polyvinyl alcohol pigment compositions whichare suitable, for example, as aquarelle colours.

U.S. Pat. No. 3,904,562 describes polyvinylpyrrolidone pigmentcompositions which have advantages in coating compositions with respectto heat resistance, bleeding and gloss.

Although the coating of pigments with polar polymers has already beendisclosed, the effect of such pigments on the warping properties oncolouring of polyolefins has hitherto not been recognised.

It has been found that virtually all organic pigments and in particulardiketopyrrolopyrrole pigments are surprisingly suitable for thewarp-free pigmenting of polyolefins if they are coated directly orindirectly with a film of a polar polymer.

The invention accordingly relates to a process for the warp-freepigmenting of polyolefins wherein an organic pigment is used whoseparticles are coated on the surface with one or more films of polarpolymers.

Polar polymers which are suitable for the coating are selected, forexample, from the group comprising linear or branched homopolymers orcopolymers from the classes comprising the

acrylic polymers based on acrylic acid, methacrylic acid and/or alkylesters thereof,

polyvinyl alcohols,

polyvinylpyrrolidones,

cellulose derivatives and

maleic anhydride-styrene copolymers.

The abovementioned polymers are employed, for example, in an amount offrom 0.5 to 20% by weight, preferably from 1 to 10% by weight, based onthe pigment.

The abovementioned acrylic polymers can, if desired, be modified bygenerally conventional methods by incorporating derivatives of theacids, for example amides, or by using further comonomers, such asstyrene, acrylonitrile, vinyl acetate, vinylphosphonates, vinylpropionate, vinyl chloride, itaconic acid, maleic acid and derivativesthereof or other α,β-unsaturated compounds (cf., for example, UllmannsEncyclop{umlaut over (a)}die der technischen Chemie [Ullmann'sEncyclopaedia of Industrial Chemistry], Volume 19, pp. 1 ff., inparticular p. 7, 4th Edition).

Suitable alkyl esters of acrylic and methacrylic acids to be employedaccording to the invention are expediently the methyl, ethyl, n-propyl,isopropyl, hydroxymethyl and hydroxyethyl esters, it also beingpossible, in addition, for smaller amounts of long-chain alkyl esters tobe present (long-chain alkyl is, for example, branched or unbranchedoctyl, nonyl, decyl, dodecyl, hexadecyl or octadecyl). Methyl esters arepreferred.

Of the polyvinyl alcohols which are of particular interest as thecoating, the generally known polyvinyl alcohols, unmodified or modifiedby esters, ethers or acetal groups, and polyvinyl alcohol-containingcopolymers, for example block copolymers with polyvinyl alcoholsegments, are suitable. Preferred polyvinyl alcohols are those having adegree of hydrolysis of greater than 80%, in particular between 85 and99.8%, and a degree of polymerisation of from 200 to 2500, in particularfrom 250 to 1800 (MW about 10,000-100,000).

The polyvinylpyrrolidones may also be modified by using copolymers suchas styrene, acrylonitrile, vinyl propionate, vinyl chloride and, inparticular, vinyl acetate. Preferred polyvinylpyrrolidones are thosehaving a molecular weight between 5000 and 500,000.

Preferred copolymers based on maleic anhydride/styrene are those havinga maleic anhydride:styrene ratio of from 1:1 to 1:3.

Preferred cellulose derivatives are sodium carboxymethylcellulose,cellulose acetobutyrate, cellulose acetopropionate,ethylhydroxyethylcellulose and, in particular, hydroxyethylcellulose.

The coating of the pigment can in principle be carried out by twogenerally known methods: by adsorption of a preformed, soluble polymeror oligomer onto the pigment or by polymerisation of correspondingmonomers in the presence of the pigment. The latter method can beemployed, in particular, when acrylic polymers are used.

In the former case, the pigment to be treated is post-treated in aqueoussuspension with the polar polymer with stirring. The starting materialmay be the dry pigment powder, a press cake or a suspension as obtainedfrom the synthesis. The polymer may either be predissolved in water oradded in solid form to the pigment suspension. If necessary, thesuspension can be dispersed by conventional methods using a dispersionunit (for example a high-pressure homogeniser or a high-speed stirrer).This is usually not necessary since the polar polymers generally havevery good wetting properties. The pigment content in the suspension canbe varied within broad limits and may be up to 40% by weight.Suspensions containing from 5 to 20% by weight of pigment areexpediently employed. The post-treatment of the pigment with the polarpolymer can be carried out at room temperature, but if desired also upto an elevated temperature of about 80° C. The stirring time variesbetween 5 minutes and 20 hours. In order to simplify filtration of thetreated pigment, filtration aids, for example from 0.5 to 15% by weightof an aliphatic 1,2-dihydroxyl compound having 8 to 22 carbon atoms, inparticular from 1 to 5% by weight of 1,2-dodecanediol, can be employedso long as they have no adverse effect on the use of the pigmentsaccording to the invention (in particular in polyethylene). Analogousadditives (for example customary dispersants) can also be added inconventional amounts to improve the dispersibility of the product Thepolymer may also be precipitated onto the pigment by adding aprecipitant (for example NaCl or Na₂SO₄) or the pigment and polymer areground together, for example in a ball or sand mill, in the presence ofa sufficient amount of solvent to at least partially dissolve thepolymer. It may also be expedient to crosslink the polymer byconventional methods. Examples of crosslinking agents which can be usedhere are boron-containing compounds (for example boric acid or sodiumborate), metal compounds (for example zirconium propylate, aluminiumsalts), and bifunctional reactants (for example dicarboxylic acids anddicarboxylic acid chlorides).

In general, the coating methods are known and described, for example, inU.S. Pat. Nos. 3,532,662, 3,806,464, 3,876,603 and 3,904,562.

In the latter case, ie. polymerisation of monomers in the presence of apigment, untreated pigment can be employed in accordance with knownmethods, as described, for example, in U.S. Pat. Nos. 3,133,893 and3,897,586, or pretreated pigment can be employed to improve theadsorption of the polymer. Various pretreatment methods of this type arecustomary and well known amongst those skilled in the art. Some of thesemethods are described, for example, in U.S. Pat. Nos. 3,544,500,3,891,572, 4,608,401, 4,680,200 and 4,771,086. In analogy to a methoddescribed in U.S. Pat. No. 3,884,871, copolymerisable monomers (forexample vinyl monomers containing polar groups, for example —COOH,—SO₃H, —NH₂, —OH, —CONH₂ etc) can also be adsorbed onto the pigment bypretreatment.

Preferred polar polymers for the process according to the invention arepolyvinylpyrrolidones, cellulose derivatives, preferablyhydroxyethylcellulose, and, in particular, polyvinyl alcohols. Thecoating is preferably in one layer.

Combinations of polar polymers can also be employed according to theinvention. Thus, for example, a further and novel pigment coating whichis advantageous for the warp-free colouring of polyolefins and whichlikewise forms the subject-matter of this invention comprises applyingan acrylic polymer film on top of a polyvinyl alcohol film adsorbed ontothe surface of the pigment particles. The preparation is carried out,for example, by adsorption of polyvinyl alcohol onto the pigment in anaqueous pigment dispersion with stirring for from 5 minutes to 4 hoursat room temperature and subsequent free-radical-initiated emulsionpolymerisation of the acrylic monomers and, if used, the comonomers inthe presence of the pretreated pigment, again with stirring, for from 1to 4 hours, but at temperatures between 60 and 100° C., preferablybetween 70 and 90° C., analogously to the abovementioned processes. Theacrylic polymer is preferably employed here in amounts of from 1 to 5%by weight, and the polyvinyl alcohol in amounts of from 0.5 to 8%, basedon the pigment.

Another novel pigment coating which is preferred for the warp-freecolouring of polyolefins and represents a further subject-matter of theinvention comprises applying the abovementioned acrylic polymer film ontop of a silane film which essentially comprises a hydrolysed silane ofthe formula

in which R is methyl or ethyl and m is a number from 1 to 6, which hasitself been adsorbed onto a film of hydrolysed zirconium acetylacetonatewhich coats the surface of the pigment particles.

The preparation is carried out, for example, as follows:

adsorption of zirconium acetylacetonate onto the pigment dispersed inwater or a lower alkyl alcohol (for example methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl, tert-butyl or amyl alcohol) withstirring, generally for at least one hour at temperatures between 20 and50° C., and subsequent hydrolysis by increasing the pH to 8-9 by addinga base, for example sodium hydroxide solution, potassium hydroxidesolution or ammonia, analogously to the method described in U.S. Pat.No. 4,880,472;

application of the silane, possibly dissolved in a lower alkyl alcohol(for example as described above) with stirring for from 1 to 4 hours atfrom 50 to 100° C., preferably 60 to 80° C; and

polymerisation of the acrylic monomers and, if used, the comonomers inthe presence of the thus-pretreated pigment by the process describedabove.

The zirconium acetylacetonate is employed in amounts of from 1 to 5% byweight, preferably from 2 to 3% by weight, calculated as the oxide, andthe silane is employed in amounts of from 1 to 10% by weight, preferablyfrom 2 to 5% by weight, in each case based on the pigment.

Preference is given to the silane of the formula

In the abovementioned methods, the polymerisation of the unsaturatedmonomers is carried out by various conventional methods, for example inaqueous emulsion using persulfate as initiator. It is of course alsopossible to use other polymerisation processes (for example in solvents)and other widely used initiators, it being possible for thepolymerisation to be initiated thermally or photochemically (cf., forexample, Comprehensive Polymer Science (Pergamon Press), Vol. 3+4, orUllmann, Vol. 19, pages 1-15).

Examples of pigments which are particularly suitable for the processaccording to the invention are those from the azo, diketopyrrolopyrrole,perylene, quinacridone, phthalocyanine, perinone, quinophthalone,isoindolinone, isoindoline, benzimidazolone, dioxazine, anthraquinone,thioindigo, methine, azomethine and metal complex series.

However, particularly important pigments are those from the azo,diketopyrrolopyrrole, perylene, quinacridone, phthalocyanine,isoindoline and isoindolinone series, preferably thediketopyrrolopyrroles and in particular1,4-diketo-3,6-di(4-chlorophenyl)pyrrolo[3,4-c]pyrrole.

The great and surprising advantage of the process according to theinvention is that polyolefins, in particular high density polyethylenes,can be coloured using high-fastness organic pigments which were hithertounsuitable or at least less suitable for this purpose.

The effect on warping of polyolefins by a pigment is tested on aninjection moulding in the form of a plate. After ageing for 72 hours at100° C., the dimensions of the plate (length and width) are measured andthe warp ΔL (length) and ΔW (width) are determined in °/oo in accordancewith the following equations:${\Delta \quad L} = {\frac{{L\quad {of}\quad {uncoloured}\quad {test}\quad {specimen}} - {L\quad {of}\quad {pigmented}\quad {test}\quad {specimen}}}{L\quad {of}\quad {uncoloured}\quad {test}\quad {specimen}} \times 1000}$${\Delta \quad W} = {\frac{{W\quad {of}\quad {uncoloured}\quad {test}\quad {specimen}} - {W\quad {of}\quad {pigmented}\quad {test}\quad {specimen}}}{W\quad {of}\quad {uncoloured}\quad {test}\quad {specimen}} \times 1000}$

Pigments which have proven to be warp-free in practice are those inwhich the absolute values of ΔL and ΔW are less than 5; values less than3 are particularly preferred.

The examples below illustrate the invention.

EXAMPLE 1

28 g of a 36% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)pyrrolo[3,4-c]pyrrole are dispersed in100 ml of water at room temperature for 45 minutes by vigorous stirring.1 g of polyvinyl alcohol ®PVA 15000 (FLUKA AG), degree of polymerisation300, degree of hydrolysis 86-89%, is subsequently added. The suspensionis stirred at room temperature for 18 hours and subsequently filtered,and the product is dried at 80° C. and powdered.

EXAMPLE 2

The procedure is as in Example 1, but the PVA 15000 is replaced by 1 gof polyvinyl alcohol ®PVA 72000 (FLUKA AG), degree of polymerisation1600, degree of hydrolysis 97.5-99.5%.

EXAMPLE 3

35.5 g of a 42.2% aqueous press cake of β-copper phthalocyanine pigmentCI Pigment Blue 15:3 are dispersed in 113 ml of water by stirringovernight using a toothed disc mill in the presence of 1.5 g ofpolyvinyl alcohol ®PVA 15000 (FLUKA AG). The product obtained issubsequently filtered, washed successively with water and acetone anddried in a vacuum drying oven.

EXAMPLE 4

15 g of the azo pigment CI Pigment Red 166 are dispersed in 134 ml ofwater and 0.75 g of polyvinyl alcohol ®PVA 15000 (FLUKA AG) using atoothed disc mill, and the mixture is subsequently stirred for a further17 hours. The product obtained is filtered, washed with water and driedin a vacuum drying oven at 80° C.

EXAMPLE 5

15 g of isoindolinone pigment CI Pigment Yellow 110 are dispersed in 133ml of water using a toothed disc mill. 1.5 g of polyvinyl alcohol ®PVA15000 (FLUKA AG) as a powder are then added with stirring. Thesuspension is stirred overnight. The product obtained is subsequentlyfiltered, washed successively with water and acetone and dried in avacuum drying oven.

EXAMPLE 6

5 g of the α-quinacridone pigment CI Pigment Violet 19 are dispersed forone hour by vigorous stirring (®Ultraturrax) with 45 ml of water and0.25 g of polyvinyl alcohol ®PVA 15000 (FLUKA AG). 50 ml of a 1M Na₂SO₄solution are subsequently added over the course of 30 minutes. Thesuspension is then filtered, and the residue is washed with water, driedat 80° C. and screened.

EXAMPLE 7

5 g of the quinacridone pigment CI Pigment Red 202 are dispersed for onehour by vigorous stirring (®Ultraturrax) in 45 ml of water with 0.25 gof polyvinyl alcohol ®PVA 15000 (FLUKA AG).The suspension is warmed to80° C., and 0.105 g of boric acid dissolved in 10 ml of water are thenslowly added dropwise, and the pH is subsequently increased to 8.5 using1N NaOH. The mixture is stirred for 2 hours and filtered, and theresidue is washed with water, dried at 80° C. and screened.

EXAMPLE 8

800 g of a 39.4% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole and 15.75 g ofpolyvinylpyrrolidone (K15, FLUKA AG) are dispersed in 2665 ml of waterby vigorous stirring (®Ultraturrax) at room temperature for 20 minutes.The suspension is stirred at room temperature for 16 hours andsubsequently filtered, and the residue is dried at 80° C. and powdered.

EXAMPLE 9

The procedure is as described in Example 8, but the 800 g ofdiketopyrrolopyrrole pigment press cake are replaced by 315 g of theisoindolinone pigment CI Pigment Yellow 110 in powder form.

EXAMPLE 10

20 g of the β-copper phthalocyanine pigment CI Pigment Blue 15:3 and 1 gof polyvinylpyrrolidone (K15, FLUKA AG) are dispersed in 200 ml of waterand 20 ml of ethanol by vigorous stirring (®Ultraturrax) at roomtemperature for 20 minutes. The suspension is stirred at roomtemperature for 16 hours and subsequently filtered, and the residue isdried at 80° C. and powdered.

EXAMPLE 11

80.2 g of a 37.4% aqueous press cake of1,4diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole are dispersed in250 ml of water with 1.5 g of polyvinylpyrrolidone-vinyl acetatecopolymer ®PVP/VA 1735 (GAF) by vigorous stirring (®Ultraturrax) at roomtemperature for one hour. The suspension is stirred at room temperaturefor 18 hours and subsequently filtered, the residue is dried at 80° C.and powdered.

EXAMPLE 12

38 g of a 39.4% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole and 0.75 g ofhydroxycellulose (medium viscosity 1, FLUKA AG) are dispersed in 91 mlof water by vigorous stirring (®Ultraturrax) at room temperature, andthe mixture is stirred for 19 hours. The suspension is subsequentlyfiltered, and the residue is washed with water and dried at 80° C. in avacuum drying oven.

EXAMPLE 13

76 g of a 39.4% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole and 1.5 g ofsodium carboxymethylcellulose (low viscosity, FLUKA AG) are dispersed in222 ml of water by vigorous stirring (®Ultraturrax) for 2½ hours. Thesuspension is subsequently stirred for 20 hours and then filtered, andthe residue is washed with water, dried at 80° C. in a vacuum dryingoven and screened.

EXAMPLE 14

25.4 g of a 39.4% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole are dispersed in65 ml of water and 10 g of a 4% ammoniacal solution of styrene-maleicanhydride copolymer (SMA 1440, Atochem) by vigorous stirring at roomtemperature for 2 hours. The pH is then adjusted to 6 using 0.1N HCl.The suspension is then stirred for a further two hours and subsequentlyfiltered, the residue is washed with water, dried at 80° C. andpowdered.

EXAMPLE 15

40.1 g of a 37.4% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole and 15 g of a 5%aqueous-ethanolic solution of methyl methacrylate-methacrylic acidcopolymer are dispersed in 95 ml of water by vigorous stirring at roomtemperature, the mixture is then stirred for 19 hours. The suspension isthen filtered, and the residue is dried at 80° C. and screened.

EXAMPLE 16

The procedure is as described in Example 15, but the methylmethacrylate-methacrylic acid copolymer is replaced by the same amountof styrene-acrylic acid copolymer.

EXAMPLE 17

28 g of a 36% aqueous press cake of1,4-diketo-3,6-di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole are stirred for2 hours at room temperature in 70 ml of water containing 0.5 g ofpolyvinyl alcohol ®PVA 15000 (FLUKA AG). 30 ml of a 0.67% emulsion ofmethyl methacrylate, 30 ml of a 0.83% aqueous solution of methacrylicacid and 25 ml of a 1.25% aqueous solution of potassium persulfate aresubsequently added under nitrogen, with the mixture being stirred for 8minutes after each addition. The suspension is heated to 80° C. andstirred at this temperature for 3 hours. The mixture is subsequentlycooled and filtered, and the residue is washed with water and dried at80° C. in a vacuum drying oven.

EXAMPLE 18

28 g of a 36% aqueous press cake of1,4-diketo-3,6di(4-chlorophenyl)-pyrrolo[3,4-c]pyrrole are suspended in100 ml of water containing 0.8 g of zirconium acetylacetonate, and themixture is stirred for 2 hours. The mixture is subsequently heated to75° C. and the pH is adjusted to 8.5 by adding 2.4 g of 1N sodiumhydroxide solution. The suspension is then stirred at 70° C. for afurther 8 hours and subsequently cooled. The pH is now 7.0. 0.25 g ofmethacryloxypropyltrimethoxysilane, dissolved in 10 ml of isopropanolare added, and the suspension is heated to 75° C. and stirred for 3hours. 10 ml of an aqueous 4% emulsion of methyl methacrylate, 10 ml ofa 5% aqueous solution of methacrylic acid and finally 10 ml of anaqueous 0.5% potassium persulfate solution are subsequently added undernitrogen. The mixture is stirred at 70° C. for 5 hours, cooled andfiltered, and the residue is washed and dried at 80° C. in a vacuumdrying oven.

EXAMPLE 19

2 g of the coated pigment obtained as described in Example 1 are mixedwith 1 kg of polyethylene ®Stamilan 9089V in a tumble mixer for 10minutes in dry form. The mixture is subsequently extruded twice at 200°C. in a single-screw extruder. The granules obtained in this way areconverted into plates measuring 174×49×2.5 mm at 240° C. in aninjection-moulding machine. After ageing for 72 hours at 100° C. thedimensions of the pigmented plate according to the invention and of apigmented plate produced under identical conditions, but containing thesame amount of uncoated pigment, are measured, and the warp effect isdetermined by the method described above.

Compared with the plate coloured using uncoated pigment, the plateproduced by the process according to the invention gives greatly reducedΔL and ΔW values.

If the above procedure is repeated, but using the same amount of one ofthe pigments obtained as described in Examples 2 to 18, a plate isobtained with analogously good ΔL and ΔW values.

What is claimed is:
 1. A process for warp-free pigmenting ofpolyolefins, which comprises: (a) coating organic pigment particles,which are diketopyrrolopyrroles, with one or more films of a preformedpolymer by adsorption of from 1 to 5% by weight of the polymer, based onthe pigment, onto the surface of the pigment particles at roomtemperature, the polymer being selected from the group consisting ofpolyvinylpyrrolidone homopolymers and copolymers; and (b) admixing thecoated pigment particles with a polyolefin.
 2. A process for thewarp-free pigmenting of polyolefins, which comprises: (a) coatingorganic pigment particles, which are diketopyrrolopyrroles, with one ormore films of a preformed polymer by adsorption of 1 to 10% by weight ofthe polymer, based on the pigment, onto the surface of the pigmentparticles at room temperature, the polymer being apolyvinylpyrrolidone-vinyl acetate copolymer, and (b) admixing thecoated pigment particles with a polyolefin.
 3. A process according toclaim 1, wherein a polyvinylpyrrolidone having a weight-averagemolecular weight M_(r) of between 5000 and 500,000 is used.
 4. A processaccording to claim 1, wherein the polyolefin is high densitypolyethylene.
 5. A high-molecular-weight organic material pigmented by aprocess according to claim
 1. 6. A process according to claim 2, whereinthe pigment is 1,4-diketo-3,6-di(4-chlorophenyl)pyrrolo[3,4-c]pyrrole.